Chapter 11: Distribution System Supplemental Information

11.4.1. Duct Fittings and Pressure Drops

11.4.1.1. General Rules

11.4.1.2. Fans and System Effect

11.4.1.3. Duct Fittings

11.4.1.4. Flex Duct

11.4.1.5. Manual Balancing Dampers

11.4.2. Fire and Smoke Dampers

11.4.3. Air Hammer

11.4.4. Duct Leakage

11.4.5. Duct Insulation

11.4.6. Indoor Air Quality

11.4.7. Under floor plenums

11.4.7.1. Displacement Ventilation

11.4.7.2. Under floor air distribution

11.4.7.3. Leakage, Drainage, Cleanliness, and Equipment Access

 

Figures

Figure 11.1: Pressure Losses Through Different Riser Connection Fitting Designs

Figure 11.2:SMACNA Design Options, all with Different Pressure Loss Coefficients

Figure 11.3: Pressure Losses Associated with Different Fan Discharge Conditions

Figure 11.4: The Impact of an Expanding Elbow on System Pressure Loss

Figure 11.5: The Impact of Fitting Proximity to Each Other on Overall Pressure Loss

Figure 11.6: Two Different Manual Damper Handles with Different Locking Arrangements

Figure 11.7: Airfoil Blade vs. Nonairfoil Blade Damper Pressure Drops

Figure 11.8: Magnitude and Rise Time for an Air Hammer Generated Pressure Pulse

Figure 11.9: Duct and Damper Damage due to Air Hammer

Figure 11.10: Precision Metering Valve and Restrictor Fittings

Figure 11.11: Fusible Links Before (top) and After (bottom) Failure

Figure 11.12: Pressure Relief Door Product Offerings and A Typical Installation

Figure 11.13: Duct Leakage Testing Machine

Figure 11.14: Protected and Unprotected Duct During Construction

Figure 11.15: Typical Under Floor Plenum Piping, Duct, and Conduit Installation

 

Tables

Table 11.1: Different Options for Moving 10,000 cfm at a Nominal 0.2 in.w.c./100 ft. Two Inch Pressure Class Duct

Table 11.2: Velocities and Velocity Pressures in Small vs. Large Ducts at Equal Friction Rates

 

This section provides supporting information for the functional testing and design review concepts described in the overview tables in Section 11.2 Commissioning the Distribution System. Energy saving opportunities, indoor air quality protection, and system safety cautions for the distribution system are presented to help commissioning providers identify and solve problems in the field. Since many of the issues discussed here involve distribution system design, designers and commissioning providers can use this information during the design process and design review to aid in preventing future operating problems.

11.4.1. Duct Fittings and Pressure Drops

The design and fabrication of the duct fittings in an air handling system can have a significant impact on the static pressure requirements for a system and, therefore, a significant impact on energy consumption and its ability to meet the design intent.

Figure 11.1: Pressure Losses Through Different Riser Connection Fitting Designs

(Courtesy of the Energy Design Resources Design Details Design Brief)

A subtle difference in the design of a fitting can have a large impact on its pressure drop. The challenging nature of the problem is often complicated by the fact that duct systems are often not well detailed. Even with adequate details, the fittings are fabricated in a three-dimensional world from representations on two-dimensional plans. In many instances, the subtle detail that significantly affects the fitting pressure loss is not apparent when viewed in plan. The 45° entry fitting shown in Figure 11.1 is a good example. When viewed in plan, the portion of the fitting that angled up at 45° would simply show up as a line, if it were shown at all. A sheet metal worker would need to understand the requirement for this type of connection from some other aspect of the construction documents or simply based on their knowledge of good practice. If they missed it and made a straight tap, they could easily double the static pressure requirements to move air into the branch from the riser, as can be seen from the graph.

The commissioning provider is in a good position to proactively promote the installation of low-pressure drop fittings through the design and construction process. During design review, the commissioning provider can help the designer understand how to best depict the fitting designs selected for the project in order to ensure their successful implementation in the field. Based on past experience and field testing, the commission provider may also be able to flag potential problem areas like the riser connection shown in Figure 11.1 and help the design team come up better solutions before the project ever reaches the field.

During the construction process, the commissioning provider can help ensure that the designer’s goals are realized by observing the construction and interacting with the craftsmen to help them understand the intent of the design. Most craftsmen will want to provide a better product if they are given some guidance in the process and can implement the improvement within the constraints of their construction budget. Many of the improvements associated with good fitting design can be realized by simply folding the same piece of sheet metal in a different manner with little if any measurable increase in cost. In some cases, the improved fittings will reduce the pounds of sheet metal required.

Finally, during the start-up and functional testing of the project, the commissioning provider can target tests to verify that the systems are delivering air at or below the static pressures intended by the design. In addition, they can carry the lessons learned from these field tests back into the design process, further improving subsequent projects. The following sections target key areas for reducing system static and energy requirements. Additional information can be obtained by logging onto www.energydesignresources.com and downloading the following design briefs (located under Publications / Design Briefs), which are available at no cost:

·       Design Details This brief illustrates how paying attention to the details of system design can yield big benefits in terms of first cost, operating cost, and improved performance.

·       Design Review This brief provides guidance and techniques that can be used for design and construction review.

·       Field Review The final brief in the series discusses approaches that can be used in the field to make sure that the design details are properly implemented by the trades people who fabricate the project.

11.4.1.1. General Rules

There are a few general rules with regard to duct fitting losses. The most important is that the duct fitting loss is a function of the velocity pressure in the duct, and that duct velocity pressure is a function of the square of the flow rate. In practical terms, it means that the pressure losses associated with a poor fitting in a low velocity duct system will be much less than the losses associated with the same fitting in a higher velocity duct system.

A second important rule is that the ratio of perimeter to cross sectional area for a large duct is generally much smaller than it is for a small duct. In practical terms, the velocities in a large duct will be much higher than they are in a smaller duct when designed at equal friction rates. As a result, the potential for a poor fitting to cause a static pressure problem is much higher in the larger ducts associated with an air handling system. Table 11.1 and Table 11.2 illustrate some of these effects.

Table  11.1: Different Options for Moving 10,000 cfm at a Nominal 0.2 in.w.c./100 ft. Two Inch Pressure Class Duct

The bottom line is that efforts to correct fitting design problems that are targeted at the duct mains will yield the most benefit for the effort and resources spent. Velocities in the mains and major duct branches in most air handling systems run in the 1,500 to 2,500 fpm range. The velocity pressures associated with these velocities are 0.15 to 0.39 inches w.c. The loss coefficients for most fittings will be direct multipliers of these numbers. In contrast, the velocities in the smaller distribution ducts serving small terminal equipment and diffusers are often below 800 fpm or a velocity pressure of 0.04 inches w.c. Thus, the difference between a poor elbow with a loss coefficient of 0.9 vs. a well-designed elbow with a loss coefficient of 0.15 may only save 0.03 inches w.c. in the low velocity network leading up to a diffuser. That same improvement in fitting design located in the main supply duct where the velocities might be running at 2,500 fpm could save 0.29 inches w.c. This static pressure savings translates into a 1/2 to 3/4 horsepower savings on a 10,000 cfm system depending on the motor and fan efficiency.

Table 11.2: Velocities and Velocity Pressures in Small vs. Large Ducts at Equal Friction Rates

Figure 11.2:SMACNA Design Options, all with Different Pressure Loss Coefficients

(Images from the SMACNA Duct Construction Standards for Low Pressure Systems)

Finally, simply specifying SMACNA construction standards will not guarantee a good fitting solution from a pressure drop standpoint. Figure 11.2 illustrates only a few of the wide variety of elbow and fitting designs included in SMACNA standard. The standard provides for a variety of fitting configurations and then allows the contractor to tailor what they use to the design requirements of the project. The design documents need to be clear on the design requirements in order to help the contractor select the best fitting design for a given application.

11.4.1.2. Fans and System Effect

As can be seen from Figure 11.3, the orientation of a fan’s discharge relative to the system that it serves can have significant impact on its performance.

Figure 11.3: Pressure Losses Associated with Different Fan Discharge Conditions

(Courtesy of the Energy Design Resources Design Details Design Brief)

Similar effects can occur with poor inlet arrangements. By monitoring and participating in the design, shop drawing review, and construction process, the commissioning provider can help ensure that the fan orientations are optimized for the project conditions.

11.4.1.3. Duct Fittings

Figure 11.4 is another example of how a common fitting geometry can cause significant pressure loss problems. Often, a designer working on a plan encounters a situation where a duct needs to be relatively square ahead of a turn, but wide and flat after the turn. An obvious solution to the geometry problem is to increase the width in the turn and then reduce the depth after the turn.

But, as can be seen from Figure 11.4, this geometry can impose a significant pressure loss penalty on the system due to the turbulence created by the expansion in the elbow. The better geometry is to make the dimension change with a standard elbow, followed by a transition. This configuration may actually cost less to fabricate since it involves the same number of fittings but may involve less sheet metal.

Another interesting and common phenomenon is the interaction of two closely spaced fittings. The distorted velocity profile exiting the first fitting results in the performance of the second fitting being worse than predicted. As a result, the overall loss through the combination is higher than projected by the fitting coefficients if no correction is made for their close proximity, as illustrated in Figure 11.5.

Figure 11.4: The Impact of an Expanding Elbow on System Pressure Loss

(Courtesy of the Energy Design Resources Design Details Design Brief)

Figure 11.5: The Impact of Fitting Proximity to Each Other on Overall Pressure Loss

 (Courtesy of the Energy Design Resources Design Details Design Brief)

11.4.1.4. Flex Duct

Improperly applied and supported flex duct can cause numerous operating problems. The sagging duct between supports that are spaced too far apart in effect adds close-coupled elbows to the system. Sagging aggravates the loss coefficient associated with the flex duct itself, which is often high relative to an equivalent diameter sheet metal duct due to the rougher interior surface conditions. It is also easy to make a very sharp turn with the flex duct, which may solve a space problem but can result in a very high pressure loss at the turn. These problems can be alleviated to some extent by:

·       Minimizing the use of flex duct to the extent possible.

·       Properly supporting the duct to minimize sagging.

·       Making any turns with gradual radius to minimize bend losses.

·       Oversizing the duct relative to an equivalent sheet metal size in situations where long runs or sharp turns are unavoidable.

All of these items are good to watch for during the construction observation associated with the commissioning process as well as when troubleshooting a system with high pressure loss problems in a retrocommissioning environment.

11.4.1.5. Manual Balancing Dampers

Manual balancing dampers are another seemingly inconsequential and passive item that can cause in significant energy waste and performance problems if the dampers do not retain their balanced settings. Losing balancing settings can cause the following problems:

Figure 11.6: Two Different Manual Damper Handles with Different Locking Arrangements

·       Comfort complaints, which usually translate into labor cost in addition to any associated wasted energy cost.

·       Higher energy consumption due to higher airflows and the possibility for unnecessary reheat. The energy waste can be particularly problematic on a constant volume reheat system where a damper vibrating open and increasing air flow to a space can be totally masked by the reheat process and therefore go undetected for years.

·       Loss of pressure relationships, which can be critical in health care and manufacturing applications. In a hospital, an improper pressure relationship can compromise patient safety. In a clean room, an improper pressure relationship can lead to contamination of the product and a loss of tens of thousands of dollars.

Figure 11.6 illustrates to different types of manual damper handles commonly found in HVAC systems. Notice how the handle on the right locks by tightening the wing nut located a significant distance away from the shaft it is serving. This configuration gives the locking mechanism a good mechanical advantage over the forces generated on the shaft by the air flowing past the damper blade. Contrast this with the arrangement for the handle on the left, which achieves the locking action by friction between the nut, handle and support bracket, all of which are concentric with the damper shaft. Experience has shown that, when applied at velocities above 800 to 1,000 fpm and/or to large damper blades, the handle style on the right will have a much better chance of maintaining is original setting compared to the handle style on the left. Often, this benefit can be obtained by simply asking the contractor to provide the handle style on the right where appropriate as long as the request is made before materials are ordered and the duct is fabricated. Including this requirement in the contract documents will guarantee that the more persistent approach is used and provide the leverage needed to motivate a change if a problem is encountered during commissioning.

11.4.2. Fire and Smoke Dampers

Figure 11.7: Airfoil Blade vs. Nonairfoil Blade Damper Pressure Drops

(Image courtesy of the Ruskin catalog)

Fire and smoke dampers are an integral part of most distribution systems and are critical links in the life safety systems and cycles associated with most buildings. Ensuring their functionality is an important aspect of commissioning the project, even though it may not directly affect system efficiency, which is often the real target of many commissioning efforts.

But, there is an efficiency-related aspect of fire and smoke damper selection that can have a measurable impact on energy consumption that is related to their pressure drop. Most fire and smoke dampers are a permanent fixture in the airflow path and thus represent a permanent pressure drop on the system. Selecting the dampers to minimize their pressure drop can often achieve measurable energy savings. Generally, there are two things that can be done to minimize the pressure drop associated with fire and smoke dampers.

1   Curtain type fire dampers can be provided in configurations that allow the folded blades to be totally out of the air stream in the retracted position. If sufficient space exists above the damper in its installed location to permit this, then providing this type of damper can reduce the pressure losses associated with the installation compared to an equivalent damper design that places the retracted blades in the air stream.

2   Smoke dampers and combination smoke and fire dampers nearly always have shaft-mounted blades that rotate around the blade axis when actuated, very similar to conventional control dampers. By providing airfoil type blades in these applications in place of the more conventional flat plate blade, significant operating savings can be realized. These pressure drops are shown in Figure 11.7.

Both of these energy saving features will most likely have some small incremental first cost associated with them. However, in most cases, the energy savings will pay for any added first cost in the first years of operation. Deferring this energy savings feature for retrofit would require removal of the existing dampers and installation of the new dampers, and the cost would be several orders of magnitude above the incremental cost difference associated with installing the energy efficient damper in the first place.

11.4.3. Air Hammer

A supply air duct high static pressure switch set at the duct pressure class rating shuts down and locks out fans to protect the air distribution system from excessive pressures that could damage ducts. This type of switch will protect the equipment from a relatively gradual over pressurization or continued operation at an over pressurized condition. Over-pressurization is created by a restriction in airflow that pushes the fan up its operating curve, and the peak on the operating curve is higher than the rated static pressure of the duct system. An example of a situation where a high static pressure switch might be desirable would be a large system that was equipped with smoke isolation dampers where the potential exists for the fan to start with the isolation dampers closed.

High static pressure switches will not protect a system from the effects of air hammer that can occur when fire dampers or smoke isolation dampers close suddenly with the system in operation. A fan that is moving approximately 25,000 cfm is literally moving a ton (2,000 pounds) of air a minute. In a typical commercial duct system, this air is moving at a velocity of 1,500 to 2,000 fpm or more, which correlates to a speed of 17 to 23 mph. A spring-loaded fire damper or pneumatically actuated smoke damper can close very quickly, often in a matter of seconds. If the dampers close quickly, the sudden stoppage of the moving air can generate pressures well in excess of the peak on the fan curve in fractions of a second, as can be seen from Figure 11.8 Note the high magnitude of the pressure pulse created, (both positive and negative) and the short pressure rise time. The pressures will be positive upstream of the damper as the air piles up against it and negative down stream of the damper due to the piston effect created by the mass of air moving away from it. Damage to the damper can be significant as shown in Figure 11.9.

Figure 11.8: Magnitude and Rise Time for an Air Hammer Generated Pressure Pulse

 
   


Figure 11.9: Duct and Damper Damage due to Air Hammer

 

(Image courtesy of the Ruskin Catalog)

Potential for Air Hammer Damage

Protecting the system from this problem can be difficult, but it is not impossible. The first step is to determine the potential for the problem to occur. Exact solutions are difficult, but by evaluating a few key criteria, one can usually assess the risk and decide if any action is necessary. Items to consider are:

·       Are the System Flow Rates and/or Velocities High? Since air hammer depends on air velocity and flow, it tends to be a more likely problem for systems moving large volumes of air or air at high velocities. Large volumes of air equates to large masses in motion with significant inertia due to the mass. High velocity air has inertia due to the rate of movement. Thus, a 2,000 cfm supply system with duct velocities in the 1,000 to 1,500 fpm range may not represent much of a risk of failure due to air hammer. But a 2,000 cfm process exhaust system with duct velocities running in the 3,000 to 4,000 fpm range to entrain vapor and particulate matter may have a high risk of failure. Systems that have both large mass flow rates and high velocities represent the biggest risk, especially if other risk factors are present.

·       What is the Percentage of Total Flow System Flow That Could Be Stopped by the Sudden Closure of Any One Damper? The systems that are the most susceptible to air hammer move most of the air through one damper assembly prior to branching out to serve different spaces. If a triggering event were to occur[1] in such a system, it would cause all of the air flow to suddenly stop, and there would be no branch ducts to act as pressure relief paths for the pressure pulse that was created. If the duct system branches ahead of the problem dampers and some of the branches do not have fire dampers, at least in the immediate vicinity, then it is less likely that all of the airflow will be simultaneously stopped with no open branches to relieve the pressure pulse.

·       What are the implications for ease of repair and loss of service? The potential penalties of taking the risk of duct failure and responding to the failure must be compared to taking measures to prevent the problem. The difficulty repairing the section of ductwork and the implications of an extended loss of service for the load served by the system should be considered. For systems where the length of duct at risk is relatively short, the duct is reasonably accessible for repair, and the implications of losing the ability to move air are not severe, the best strategy may be to simply accept the risk and minimize the potential for an event through a proactive inspection and maintenance program. In this case, a loss in service may mean some discomfort or loss of control in the space but will not place life, machinery, or product at risk, or create an excessive exposure to liability. A system where the duct at risk is a short run located in an accessible mechanical room serving an office environment where no lawyers or surgeons work is a good example of such a system.

On the other hand, if the system contains a long run of duct at risk in a difficult to access location and/or serves a load where loss of airflow could place life or machinery at risk, result in a significant loss of product, or if it is a system serving surgeons or lawyers, then taking steps to minimize or prevent a duct explosion or implosion may be highly desirable. Systems serving clean rooms or surgical suites are good examples of this type of system. Systems serving offices in a high rise building where the air handling equipment is on one level and the duct system leaves the unit and travels to a different floor in a fire-rated chase with fire or smoke dampers leaving the chase is another good example.

Protecting a System from Air Hammer

If you are working with a system that meets one or more of the risk criteria listed above, then it would be advisable to explore options that will help protect the system from air hammer. It is fairly unlikely that the system will see a fire or smoke damper closure due to and actual fire or smoke condition during its life, but it is highly likely that it will see an accidental closure of a fire or smoke damper at some point due to a false alarm or linkage failure. The likelihood is especially high during start-up and testing. There are several measures that can be used to protect the system from air hammer.

Figure 11.10: Precision Metering Valve and Restrictor Fittings

Both of these devices can be used to slow down the actuation time of a pneumatic smoke damper. The metering valve on the left allows the rate of closure to be precisely adjusted, while the restrictor fittings on the right allow no adjustment but also are much less expensive. The inline check valve (lower right) can be piped in parallel with any of the restrictors to allow quick opening/slow closing action to be achieved.

1   Restrict the Operating Speed of Smoke Dampers This measure is relatively easy to implement and is probably a good idea on any smoke damper. Most codes allow for up to two minutes for the closure time for a smoke or combination smoke/fire damper. Taking advantage of this time limit will go a long way towards preventing air hammer problems in the duct system.

Electrically driven smoke dampers will have their operating speed restricted to some extent simply due to the operating time associated with a typical electric actuator. Typically, they will take at least 15 seconds to go full stroke and can take 30-60 seconds or more.

Pneumatic actuators tend to be very rapid and must be slowed down by restricting the air flow to the cylinder. In-line restrictor fittings can be installed in the line to each actuator or in the air mains to the actuators at the panel. Installing the fitting in the panel makes it more accessible and minimizes the number of restrictors. However, it does not protect the system from sudden damper closures due to a pneumatic line failure from the panel to the damper assembly. Installing the restrictor fitting at the actuator location immediately ahead of the actuator will avert this problem and provide the safest installation.

Restrictor fittings are available with a variety of port sizes from most of the control system manufacturers. It may take field testing and experimentation to find the correct orifice size for a project to provide the best rate of closure. Another option is to use a micrometer-type needle valve available from many of the precision valve manufacturers. The needle valve will allow the rate of closure to be custom tailored to the project, but is significantly more expensive since these metering valves are manufactured for process requirements.

In either case, a quick opening, slow closing feature can be provided by piping a check valve in parallel with the restrictor so that air can flow to the actuator quickly to open the damper, but must flow through the restrictor when it is bled from the actuator to close it. Some of the process grade metering valves have this feature built into them allowing both functions to be obtained in one assembly.

Figure 11.11: Fusible Links Before (top) and After (bottom) Failure

Most fusible links are simply two pieces of metal connected with solder with a very specific melting temperature. There are also electrical switches that use a bimetallic sensing element to provide the fusible link function. These devices are especially useful for electrically actuated combination fire and smoke dampers. At least one manufacturer that offers a manually resettable link uses a bimetallic trigger, avoiding the need to stock replacement links to address random failures.

2   Install Fusible Links With Temperature Ratings Higher than the Maximum Temperature that can be Produced by the System This measure is also relatively easy to implement and should be considered on any system with fire dampers. In addition to preventing air hammer, installing the appropriate fusible links will help prevent nuisance fire damper closures triggered by non-fire events.

Fusible links come in a variety of ratings in addition to the standard 165°F rating (212°F and 285°F are common examples). Figure 11.11 shows a picture of typical links and some information about link options. In most instances, the code requires that the link be rated above the highest temperature that will be encountered under any of the systems operating modes with an upper limit of 350°F. It is important to select links that are rated above the highest temperature that will ever be encountered in the system. There are several situations that can result in system temperatures in excess of 165°F in the course of normal operation but are not recognized at the time of design or shop drawing review when the link rating can be easily changed. As a result, when these conditions are encountered in the operating system, they cause the fire dampers to release for non-life safety reasons. At a minimum, this can result in a flow outage and an emergency maintenance call to locate and reset the offending fire damper. Often, lack of an available replacement link in the repair parts stock makes this an arduous process. Common conditions that can lead to inadvertent fire damper trips include:

·       Warm-up Cycles During a warm-up cycle, air temperatures are often significantly above what would typically be encountered in the system. In some instances, the warm-up cycle fully opens the heating valve and circulates the warmest air available until the space is up to temperature. If the heating medium is of a relatively high temperature, such as low pressure steam, which can easily produce 200-230°F air if the coil is uncontrolled, the air circulated in a warm-up cycle can be above the standard fusible link rating.

·       Freeze Prevention Cycles Some systems fully open the heating valve in the air handling unit during the off cycle in an effort to prevent freezing of any water coils in the unit. When this occurs, the stagnant air inside the air handling unit casing will approach the temperature of the heating medium, which can be in the range of 180-200°F on water systems and over 240°F for steam. When the unit starts, this slug of warm air is moved out of the unit and down the duct system and can melt the fusible links on the fire dampers.

3   Radiant Temperature Effects If a reheat coil or heating coil is located in the line of site of a fire damper and its fusible link, the radiant temperature from the coil can melt the fusible link if the surface temperature of the coil is elevated above the rating of the link. Usually, this problem occurs when the air handling system is shut down but the flow to the terminal reheat coils is not shut down. Since there is no flow over the coil, the coil surface temperature and the stagnant air in the vicinity of the coil approach the temperature of the heating medium.

4   Reinforce the Duct System Upstream of the Damper to Withstand the Pressure Pulse If the duct run at risk is shorter small, then the most cost effective solution may be to simply make the duct strong enough to withstand the pressure pulse should it occur. This strategy can have added benefit by minimizing duct rumble and break-out noise.

5   Install Pressure Relief Doors Several manufacturers offer pressure relief doors designed to blow open and create a pressure relief path when the pressure difference across them exceeds the rated setpoint. Figure 11.12 illustrates some typical product offerings along with a picture of one installed in a system.

Figure 11.12: Pressure Relief Door Product Offerings and A Typical Installation

The pictures to the left illustrate one type of pressure relief door where the function is combined with the fire damper access door. The picture on the right is a different product installed in a working system. This particular product is purely a pressure relief device. This door is to relieve negative pressure and opens into the unit, so the conduit in front of it will not interfere with its operation. Notice the warning placard advising operating staff to keep clear of the area where the door will swing if it blows open. Both devices can be reset by manually reclosing the door after a trip.

 

(Left picture courtesy of the United McGill web site. Right picture by PECI)

Generally, the doors are rated for a given wide-open pressure drop at a given flow rate. When applying them in a situation where they may need to relieve the air handling system’s rated flow with the system operating (compared to a situation where a safety switch will shut down the fan on an over-pressure condition and the doors are just providing a relief path to protect the duct from a pressure pulse), they need to be sized to match the requirements of the system so that the pressure drop through the open blow-out door at the required flow rate will not exceed the rated pressure of the duct system.

The doors are released by a mechanism that applies the force created by the pressure difference acting on the door area against a spring via an over-center lever system. So, there is some minor movement of the door away from its seat and gaskets that increases as the pressure differences approaches the trip point. As a result, the doors will introduce some leakage into the system, especially at higher differential pressures. This data is cataloged by the manufacturer. The leakage should be included in the overall leakage allowance associated with the system.

Finally, when applying doors on a unit, bear in mind that the air that is being relieved ultimately needs to have an exit path. A pressure relief door that blows open to a confined and sealed area, like the service corridor built into large, custom roof top air handling systems may only transfer the over-pressure problem from the air handling unit casing to the enclosure envelope. Usually this problem can be addressed by providing a louvered and screened opening through the enclosure wall. The leakage from the doors into the service corridor coupled with the louvered opening can provide an added benefit by ventilating the service corridor, which can improve the reliability of controls, drives and other electronic equipment during extreme weather.

11.4.4. Duct Leakage

Air that leaks from the distribution duct system often represents wasted energy both in terms of the fan energy used to covey the air, and the HVAC process energy used to heat, cool, humidify, dehumidify, filter and otherwise condition the air. Therefore, controlling duct system leakage is an important step in controlling the overall energy consumption rate and efficiency of a system.

Figure 11.13: Duct Leakage Testing Machine

Most duct leakage testing machines are relatively simple devices consisting of a variable speed fan to provide the air necessary pressurization the system and calibrated orifice plates used to measure the fan’s airflow. The airflow moved by the fan to pressurize the system represents the system leakage at the test pressure produced by the fan.

It is important to keep the following points in mind:

·       A totally leak free duct system is a practical impossibility, but proper construction practice can yield reasonably leak free systems.

·       Leakage from a duct that is downstream of the terminal equipment and ends up in the conditioned space may not be critical unless it is significant enough to prevent space temperature or pressure relationships from being maintained.

·       Significant leakage from a duct system in a given area can impact pressure relationships in adjacent areas. This factor can be important in Health Care and Laboratory environments. Troubleshooting pressure relationship problems may involve some duct leakage testing to determine if duct leakage is impeding the achievement of the desired pressure relationships.

·       The leakage classification specified for a duct system should reflect the requirements of the system and the operating conditions it will see. For example, fabricating and sealing a small, low pressure duct system to a high leakage classification may not yield measurable benefits and may actually waste more time, energy and other resources than it conserves.

·       Leakage is generally related to the surface area of the sheet metal that comprises the duct system. Therefore, an extensive duct system will tend to leak a larger percentage of its total capacity when constructed to a given leakage classification than a relatively short, compact duct system constructed to the same leakage classification.

·       Existing leakage guidelines, standards, and testing techniques are generally aimed at duct leakage. The leakage from the housings and casings associated with the terminal units, air handling units, diffusers, and other devices that will ultimately be connected to the duct system can be as significant if not more significant than the leakage from a well-constructed duct system. The design leakage allowances and the specified quality criteria for this equipment need to take this into account. In some cases, independent testing of large equipment such as the air handling units or the supply plenums associated with clean rooms may be warranted.

The Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) is an excellent source of procedural and background information on duct system leakage testing. Refer to the SMACNA HVAC Air Duct Leakage Test Manual, which can be found at www.smacna.org.

11.4.5. Duct Insulation

Most supply duct systems carry conditioned air and therefore require insulation. This insulation performs two important thermal functions and can also provide some acoustical benefit. From a thermal standpoint, the insulation ensures that the conditioned air arrives at the load at the desired temperature and is relatively unaffected by heat gains or losses through the walls of the duct between the central conditioning equipment and the terminal equipment. For ductwork handling cold air, the insulation also ensures that the surface temperature of the distribution system is maintained at a temperature that is above the dew point of the ambient environment around the duct, thereby preventing condensation. The amount of insulation required to do this will vary with the ambient temperature and humidity levels and the internal conditions inside the duct. For instance, a duct that carries 46°F 95% relative humidity air to a clean room in a roof- mounted duct in Florida will need more insulation than it would once it was inside the conditioned area.

Similar considerations apply to hot ducts, but the condensation issues are reversed. For instance, if the clean room duct discussed in the preceding paragraph (which would carry humidified air in the winter) were located on a roof in a Northern climate, the winter time ambient temperatures might cool the internal surface of the duct sufficiently to condense the humidity out of the supply stream on the inside of the duct unless the insulation was sufficient to prevent this from occurring.

Since the heat gains into the duct system are related to the surface area and the heat transfer coefficients, small ducts (with relatively large surface areas for the cross section they contain - See Table 11.1 and Table 11.2) carrying air at low velocities can show surprisingly high temperature rises in short distances if they are not insulated. These ducts contain a low mass flow rate and therefore, require very little energy input to cause a temperature change. In addition, they have an extensive surface area over which to transfer heat relative to their volume. High temperature rises may occur in VAV systems operating at low flows where the ductwork downstream of the terminal unit was not insulated as a cost saving measure. Temperature rise can also be a problem on any long, low velocity, low capacity, uninsulated duct system.

When responding to a condensation problem, remember that a relatively small leak from a duct carrying cold air can create the illusions of inadequate insulation due to the localized cooling and condensation it creates. This condition can be difficult to detect on externally insulated duct systems since the point of the leak may actually be some distance from the point where the air escapes from under the insulation and causes condensation.

Hangers and other thermally conductive materials that are indirect contact with a cold duct can end up causing condensation problems due to the cooling effect they experience by direct contact with the cold duct surface. Solving this problem usually involves insulating the hangers to a point where the conduction no longer lowers the surface temperature below the ambient dew point.

Duct systems are often insulated by using a duct liner rather than external insulation. This has the advantage of minimizing labor costs since the duct is insulated during fabrication and does not require insulation in the field. It also improves the acoustical qualities of the duct, both in terms of transmitted noise and radiated noise. However, the liner does provide an environment conducive to microbiological growth and for this reason can be an indoor air quality (IAQ) concern. In certain environments, like health care and clean room systems, codes or quality control standards prohibit the use of duct liner for these reasons.

11.4.6. Indoor Air Quality

In addition to the duct liner IAQ concerns outlined in the preceding section, the condensation issues discussed previously also have IAQ implications. Condensation inside the duct can lead to internal microbiological growth. Condensation outside the duct can cause moisture problems with other building materials, leading to both degradation and microbiological activity.

Figure 11.14: Protected and Unprotected Duct During Construction

 

Cleanliness during construction relates to IAQ during occupancy. If the duct work on site becomes contaminated with water, construction dust, or debris prior to installation, it can result in air quality problems that are costly to correct at a later date. Figure 11.14 contrasts ductwork that was protected on a project with some that was not.

Operating the systems to provide temporary heat or cooling during construction may also compromise IAQ if filtering is not properly applied and maintained. During the final phases of most construction projects, considerable dust can be generated by some of the finishing operations such as drywall sanding and concrete floor leveling. This dust requires special considerations in terms of filtration that may not be addressed by the filters provided with the system for the design operating conditions. The dust from these operations is much finer than in a typical building environment and will pass through some lower efficiency filters that might be used. Therefore, adequately protecting the duct system from this dust may require using higher efficiency filters during construction than will be installed for normal operation, perhaps as high as 90-95% (ASHRAE dust spot rating). In addition, it will most likely be necessary to provide filters with a similar efficiency rating at the return and exhaust grill locations to protect the duct system from abnormal dust volumes entrained in the return and exhaust streams. Higher efficiency filtration is also advisable on the supply diffusers since there will be periods of time when the systems will be off-line or running in unusual configurations that could result in back-flow through the supply system, contaminating it from the diffuser location back.

11.4.7. Under floor plenums

Under floor supply plenums have been used in Europe for years and are starting to become common in the United States. Two subtly different cooling approaches are used with this distribution system: displacement ventilation and under floor air distribution. Both approaches utilize an under floor supply plenum as:

·       A distribution technique that is flexible and easily modified when the area served is reconfigured.[2]

·       A distribution technique that promotes a general upward flow of air through the occupied zone, thereby improving the removal of contaminants and the overall ventilation effectiveness.

·       A distribution technique that tends to require lower static pressures and therefore less fan energy as compared to a more conventional overhead air approach.

Since air is introduced at floor level under both approaches, the supply temperature generally needs to be higher than that associated with a conventional cooling approach in order to avoid drafts and occupant discomfort. This higher supply air temperature requirement can cause humidity control and condensation problems in hot and humid environments if the processes at the central air handling equipment are not designed properly to prevent it. Since both approaches are a cooling technology, perimeter heating loads need to be addressed by an independent system.

Both approaches are often combined with task/ambient cooling where an effort is made to provide each occupant with some localized control of their environment using a mini HVAC system that, at a minimum, allows them to control the rate and direction of the ventilation and cooling air injected into their space. Some systems supplement this with radiant heat capabilities, supplemental cooling capabilities, white noise systems, and task lighting control systems. Many of the systems can provide localized occupancy control of all of these parameters with an occupancy sensor arranged to be triggered only by the zone’s occupant.

11.4.7.1. Displacement Ventilation

Displacement ventilation introduces air at a near neutral temperature (at a temperature very close to the desired space temperature) into the space at a low velocity and near the floor level. As the heat gains in the space warm this air, it starts to rise. The design goal is to have the air be warmed to the design space temperature by the time it reaches the ‘occupied zone’ between 4 and 6 feet above floor level. The heat gains in the space continue to warm the air and it rises above the occupied zone. As a result, the air exits the space at temperatures that are above the design space temperature. This design approach aims at creating temperature stratification in the space. This approach is in direct contrast to the more conventional distribution approach in which the goal of the diffusers is to induce secondary air flow into a relatively cold air stream supplied to the space resulting in complete mixing and a relatively uniform space temperature throughout the space.

Displacement ventilation allows less air to be moved by the air handling system as compared to the more conventional approach, which saves fan energy. To function as a true displacement ventilation system, the supply air must be introduced at very low velocities to ensure that no mixing will occur and to promote stratification. The low velocity tends to place a cap on the maximum amount of cooling that can be handled by the approach. Some systems work around this by providing supplemental, recirculating-type cooling systems in areas with higher loads than can be met by displacement ventilation alone.

11.4.7.2. Under floor air distribution

Under floor air distribution is similar to a more conventional distribution approach since the air flow rates associated with it are high enough that mixing occurs in the occupied zone, in contrast with displacement ventilation, where the goal is to avoid mixing and promote stratification. Under floor air distribution realizes some of the improved ventilation effectiveness benefits associated with displacement ventilation since the general flow through the space is from floor to ceiling. However, the higher air change rates associated with it defeats the stratification effect promoted by displacement ventilation. The air in the occupied zone tends to be more uniform in temperature, just as it would with a conventional overhead distribution system. Since supplying air at floor level requires a higher delivery temperature than that associated with a conventional system in order to avoid occupant discomfort, the net air flow rate associated with this approach will tend to be higher than the air flow associated with an overhead system. This increased air flow tends to increase the fan energy requirements. However, the lower supply system static requirements often outweigh the increased flow requirement, and the net effect is reduced fan energy compared to a conventional overhead system.

11.4.7.3. Leakage, Drainage, Cleanliness, and Equipment Access

Regardless of the approach used, the under floor supply plenum is an integral part of the air handling system, and the integrity of the plenum is critical to system performance. Excessive leakage from the plenum will result in poor performance and energy waste. During the construction and commissioning of a project that is using this technology, extensive efforts need to be made to properly seal the plenum and maintain its cleanliness. A functional test aimed at documenting the ‘as built’ leakage rate is also advisable. In addition, remodeling and renovation projects and movement of the building over time could adversely impact the performance of the plenum. For these reasons, the following recommendations apply to systems using this distribution technique.

·       Include requirements for sealing penetrations in all specifications and work orders for remodeling or renovation projects in the area. Any opening made through the floor, an exterior wall, or an interior wall that separates the office area from adjacent areas of the building needs to be sealed around the penetration. Conduits that serve as raceways between these areas need to be sealed internally in addition to having the pipe sealed to the surrounding structure. New walls or other cavities that extend from the plenum concrete floor slab into or through the occupied zone and the ceiling plenum should have the stud cavities and any other internal passages sealed to prevent air migration between pressure zones via these spaces.

·       Periodically re-test the plenum leakage rate and compare it to the ‘as built’ leakage rate documented in the commissioning process. When and if significant changes become apparent, perform additional investigation to identify and repair leaks. The test procedure associated with making this test should be included in the commissioning documentation for the project.

·       When servicing equipment located in the raised floor area, minimize the amount of time that floor tiles are removed. The opening created by removing one floor tile can represent a significant leak in the plenum and may adversely affect system performance and comfort in other areas of the space.

Plenum cleanliness and water control are also important aspects related to the indoor air quality (IAQ) delivered by this type of system. During construction, the following precautions should be taken in this regard:

·       Floor drains or sumps and sump pumps should be installed to provide a way of removing water from the plenum. Water could enter the plenum through a variety of sources including a leak or overflow from a plumbing fixture in a space above or adjacent to it, a leak in any HVAC or other process piping running in the plenum, or a leak through the building envelope. Floor drains should be equipped with trap primers to ensure that the water seal in the traps is maintained and prevent odor and rodent problems in the plenum. It may also be desirable to equip the traps with backwater valves that will close to prevent flow out of the sewer system into the plenum in the event of a sewer line blockage.

Duct, pipe, and conduit runs should be supported on Unistrut™ or similar channels to provide a free and clear drainage path from any point in the plenum to the nearest floor drain or sump. Figure 11.15 illustrates this.

Figure 11.15: Typical Under Floor Plenum Piping, Duct, and Conduit Installation

·       Where junction boxes are secured directly to the floor, extension rings should be installed and the wiring should be arranged so all terminations and splices occurred in the extension ring, thereby elevating them above the first 1’ to 1-1/2’ of water accumulation in the event of a flood.

·       Moisture detectors should be installed and wired to the DDC system to provide an alarm in the event that water does begin to accumulate in the floor plenum.

Work orders and specifications for work in the plenum area should include language directed at maintaining these provisions. Preventive maintenance schedules should include verification of the integrity of the back water valves and the operation of the trap primers and moisture detectors.

Equipment access also needs to be considered during design when under floor plenums are used. Terminal equipment that is located in the plenum can become a nightmare to service if it is situated so that filing cabinets, desks and other office furniture must be moved in order to gain access for routine service requirements. A better approach is to locate the equipment so that it can be serviced by removing tiles that are located in the corridor and walkway areas in the office space. This can create some traffic problems in the office for the tenants when equipment is being serviced but usually is preferable to having to interfere with the productivity of a worker in an office or cubicle.



[1]   Examples of triggering events might include the following:

·       An uncontrolled blast of heat from a heating coil due to a steam valve that failed open melts the fusible links of the fire damper in the duct main leaving the equipment room.

·       The air main serving the pneumatically actuated, normally closed smoke isolation damper on the discharge of the supply fan pops off the barbed tee where it splits to the actuators, causing a sudden and total loss of actuating pressure and resulting in the nearly instantaneous closure of the dampers by the actuator springs.

[2]   In a modern office, this is an important consideration due to the high turn-over or ‘churn’ rates. When properly executed, under floor plenums allow a space to be reconfigured very quickly by simply relocating the floor tiles that contain the diffusers, electrical outlets and communications outlets to match the new requirements.